Drill bits for drilling subterranean rock formations employ cutting elements to remove the underlying earth structures. However, as drilling proceeds the cutting elements begin to wear and fracture, causing premature failure of the bit. When the cutting elements wear down to the point of needing replacement, the entire drilling operation must be shut down to replace the drill bit, costing significant time and money. It is therefore desirable to maximize the cutting elements' useful life by increasing their resistance to damage through both wear and impact.
Typical materials exhibiting suitable characteristics for use in cutting elements include refractory metals, metal carbides, such as tungsten carbide (WC), and superhard materials, such as diamond. Diamond is resistant to wear, but is brittle and tends to fracture and spall in use. Cemented WC, on the other hand, is more ductile and resistant to impact, but tends to wear more quickly than diamond. Many attempts have been made to marry the wear resistance of diamond to the impact resistance of WC in earth-boring drill bit cutting elements. Cutting elements are typically composed of a PCD layer or compact formed on and bonded under high-pressure and high-temperature conditions to a supporting substrate such as cemented WC, although other configurations are known. A binder material, such as nickel, molybdenum, cobalt, and alloys thereof, is used to cement the WC and the PCD layer together, creating a continuous matrix to hold the WC and PCD layer in place.
The outermost or working layer of such a cutting element comprises a PCD layer wherein intercrystalline bonding occurs between adjacent diamond crystals. The PCD layer has a continuous PCD phase and a continuous matrix phase throughout. Accordingly, a substantially complete and substantially intact layer of PCD would remain if the layer of PCD were leached of all binder content. To improve bonding between the PCD layer and the substrate, transition layers may be interposed between the substrate and the working layer wherein gradually increasing concentrations of PCD or diamond grit are introduced into the continuous matrix phase in each layer.